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13,14-diphenyldibenzo[b,j][4,7]phenanthroline (DBP3) in various solvents was studied by time-resolved fluorescence and fs transient absorption (fs-TA) spectroscopy. An intramolecular benzene excimer is demonstrated to form within DBP3; it exhibits strong redshifted emission with maximum at 540–640 nm. "Intrinsic" fluorescence from DBP3 is dramatically quenched down to τ = 50–400 fs in all the solvents studied. Fs-TA and time-resolved fluorescence spectra have proved that relaxed intramolecular benzene excimer is formed from S1 state via hot excimer state with three lifetime components: 50 fs, ∼3.5 ps, and ∼25 ps, which are of the inertial (electronic) and diffusive parts of the relaxation due to solute–solvent interaction. Formation of triplet states via intersystem crossing was observed directly from the upper excited electronic states of DBP3. [ABSTRACT FROM AUTHOR]

A long 0.9 ps lifetime of the upper excited singlet state in perylene is resolved by femtosecond pump–probe measurements under ultraviolet (4.96 eV) excitation and further validated by theoretical simulations of transient absorption kinetics. This finding prompts exploration and development of novel perylene-based materials for upper excited state photochemistry applications. [ABSTRACT FROM AUTHOR]

Catalytic metal hydride hydrogen atom transfer (MHAT) reactions have proven to be a powerful method for alkene functionalization. This work reports the discovery of Co‐porphines as highly efficient MHAT catalysts with a loading of only 0.01 mol % for unprecedented chemoselective allene functionalization under photoirradiation. Moreover, the newly developed bimetallic strategy by the combination of photo Co‐MHAT and Ti catalysis enabled the successful carbonyl allylation with a wide range of amino, oxy, thio, aryl, and alkyl‐allenes providing expedient access to valuable β‐functionalized homoallylic alcohols in over 100 examples with exceptional regio‐ and diastereoselectivity. Mechanism studies and DFT calculations supported that selectively transferring hydrogen atoms from cobalt hydride to allenes and generating allyl radicals is the key step in the catalytic cycle. [ABSTRACT FROM AUTHOR]

Catalytic metal hydride hydrogen atom transfer (MHAT) reactions have proven to be a powerful method for alkene functionalization. This work reports the discovery of Co‐porphines as highly efficient MHAT catalysts with a loading of only 0.01 mol % for unprecedented chemoselective allene functionalization under photoirradiation. Moreover, the newly developed bimetallic strategy by the combination of photo Co‐MHAT and Ti catalysis enabled the successful carbonyl allylation with a wide range of amino, oxy, thio, aryl, and alkyl‐allenes providing expedient access to valuable β‐functionalized homoallylic alcohols in over 100 examples with exceptional regio‐ and diastereoselectivity. Mechanism studies and DFT calculations supported that selectively transferring hydrogen atoms from cobalt hydride to allenes and generating allyl radicals is the key step in the catalytic cycle. [ABSTRACT FROM AUTHOR]

Photoreduction of CO2 to C2+ solar fuel is a promising carbon‐neutral technology for renewable energy. This strategy is challenged by its low productivity due to low efficiency in multielectron utilization and slow C−C coupling kinetics. This work reports a dual‐metal photocatalyst consisting of atomically dispersed indium and copper anchored on polymeric carbon nitride (InCu/PCN), on which the photoreduction of CO2 delivered an excellent ethanol production rate of 28.5 μmol g−1 h−1 with a high selectivity of 92 %. Coupled experimental investigation and DFT calculations reveal the following mechanisms underpinning the high performance of this catalyst. Essentially, the In−Cu interaction enhances the charge separation by accelerating charge transfer from PCN to the metal sites. Indium also transfers electrons to neighboring copper via Cu−N−In bridges, increasing the electron density of copper active sites. Furthermore, In−Cu dual‐metal sites promote the adsorption of *CO intermediates and lower the energy barrier of C−C coupling. [ABSTRACT FROM AUTHOR]

Photoreduction of CO2 to C2+ solar fuel is a promising carbon‐neutral technology for renewable energy. This strategy is challenged by its low productivity due to low efficiency in multielectron utilization and slow C−C coupling kinetics. This work reports a dual‐metal photocatalyst consisting of atomically dispersed indium and copper anchored on polymeric carbon nitride (InCu/PCN), on which the photoreduction of CO2 delivered an excellent ethanol production rate of 28.5 μmol g−1 h−1 with a high selectivity of 92 %. Coupled experimental investigation and DFT calculations reveal the following mechanisms underpinning the high performance of this catalyst. Essentially, the In−Cu interaction enhances the charge separation by accelerating charge transfer from PCN to the metal sites. Indium also transfers electrons to neighboring copper via Cu−N−In bridges, increasing the electron density of copper active sites. Furthermore, In−Cu dual‐metal sites promote the adsorption of *CO intermediates and lower the energy barrier of C−C coupling. [ABSTRACT FROM AUTHOR]

The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps via singlet fission-intersystem crossing, while the short-lived triplet states (1.5 ns) are generated via singlet fission from vibrational electronic states. In the meantime, polarons were formed from hot excitons on a timescale of <30 fs and recombined in ultrafast lifetime (0.37 ps). In addition, the characterization of hexaphenyl film suggests the morphologies of crystal and aggregate to wide applications in organic electronic devices. The present study provides a universally applicable film fabrication in hexaphenyl system towards future singlet fission-based solar cells. [ABSTRACT FROM AUTHOR]

A low-cost and easy-to-construct dye-sensitized photoanode for water oxidation was prepared by co-loading a new triphenylamine donor–acceptor organic chromophore (D1) and a Co4O4 cobalt cubane-based water oxidation catalyst (CoF) on TiO2 thin films. As the first example of a molecular catalyst-based, noble metal-free, dye-sensitized photoanode, the electrode, TiO2‖D1,CoF, produces a higher photocurrent than other noble metal-free, dye-sensitized photoanodes with a relatively high faradaic efficiency for O2 production. The success of the dyad system lies in retarded interfacial charge recombination and extended light absorption both of which take advantage of the pyridine anchor-functionalized chromophore. In addition, the CoF catalyst, decorated with hydrophobic aliphatic chains, serves as a protective layer for stabilizing the organic dyes on electrode surfaces. Transient absorption measurement suggests the accumulation of oxidative equivalents on CoF by electron transfer on the TiO2 surface. The results and design principles presented here demonstrate the use of molecular engineering as a powerful tool for the exploration of low-cost solar water-splitting devices. [ABSTRACT FROM AUTHOR]

The potential to improve the efficiency of photovoltaic devices through singlet exciton fission (SF) has raised interest of researchers during the last decade. Organic small molecules of rubrene were fabricated into spherical nanoaggregates (NAs) of diameter ∼250 nm by the re-precipitation method. J-type aggregation inside the NA with pivotal electronic coupling occurs in NAs. Observed strong fluorescence quenching indicates the presence of competing pathways in the NAs: hot and thermally activated SF, triplet–triplet annihilation (TTA), and energy transfer between inhomogeneously broadened NA ensembles. SF which proceeds directly from upper excited electronic states within 340 fs (vs 200 fs in crystals; 30 fs in films) results in a triplet quantum yield of 162%. With an aim to apply rubrene in photovoltaics, a rubrene NA-integrated film was prepared on top of the CH3NH3PbI3 layer. Triplet formation within 25 ps in the NA layer was well-documented. Subsequently, the triplet excitons are accumulated and further upconverted via TTA to populate the conduction band of the perovskite within 39 ps (80%) and ≫10 ns (20%). Our findings are crucial for understanding photophysical mechanisms of rubrene/perovskite interaction after light absorption by rubrene. [ABSTRACT FROM AUTHOR]

Acid‐catalysed Friedländer cyclization of 2‐aminobenzophenones with 1,2‐, 1,3‐, and 1,4‐cyclohexadiones, followed by aromatization, has been used to prepare dibenzophenanthrolines (DBPs) containing 1,10‐, 1,7‐ and 4,7‐phenanthroline units, whose optoelectronic properties have been compared. The LUMO energies for all isomers were similar but the 4,7‐isomers possessed significantly higher HOMOs than the other isomers, resulting in them having smaller HOMO‐LUMO energy gaps. Compared with 1,10‐DBP isomers, the emission of 1,7‐DBPs is strongly quenched due to efficient charge transfer. The 4,7‐DBP isomers show strongly enhanced photoluminescence due to excimer formation but appear to be less photostable due to their higher HOMO levels. Bis(phenylethynyl)‐substituted derivatives demonstrated improved solubility and stronger luminescence properties than their unsubstituted analogues. [ABSTRACT FROM AUTHOR]

A four‐step route for the synthesis of 5‐azatetracene (benzo[b]acridine) has been developed, employing a base‐catalysed Friedlander condensation reaction between 3‐amino‐2‐napthaldehyde and cyclohexanone as the key step followed by dehydrogenation of the intermediate. The optical and electrochemical properties of the 5‐azatetracene were investigated by UV‐vis and photoluminescence spectroscopy, and by cyclic voltammetry and compared with those of tetracene. It is found that 5‐azatetracene shows broader absorption in the visible region than tetracene, exhibits a higher luminescence quantum efficiency, and possesses a lower‐lying LUMO level and smaller HOMO‐LUMO band gap. Time‐resolved PL spectroscopy was used to elucidate the reasons for the more efficient luminescence of 5‐azatetracene. Field‐effect transistor measurements revealed the ambipolar nature of charge transport in 5‐azatetracene. [ABSTRACT FROM AUTHOR]

Perylenebisimide (PBI)–anthracene (AN) donor–acceptor dyads/triad were prepared to investigate spin–orbit charge‐transfer intersystem crossing (SOCT‐ISC). Molecular conformation was controlled by connecting PBI units to the 2‐ or 9‐position of the AN moiety. Steady‐state, time‐resolved transient absorption and emission spectroscopy revealed that chromophore orientation, electronic coupling, and dihedral angle between donor and acceptor exert a significant effect on the photophysical property. The dyad PBI‐9‐AN with orthogonal geometry shows weak ground‐state coupling and efficient intersystem crossing (ISC, ΦΔ=86 %) as compared with PBI‐2‐AN (ΦΔ=57 %), which has a more coplanar geometry. By nanosecond transient absorption spectroscopy, a long‐lived PBI localized triplet state was observed (τT=139 μs). Time‐resolved EPR spectroscopy demonstrated that the electron spin polarization pattern of the triplet state is sensitive to the geometry and number of AN units attached to PBI. Reversible and stepwise generation of near‐IR‐absorbing PBI radical anion (PBI−⋅) and dianion (PBI2−) was observed on photoexcitation in the presence of triethanolamine, and it was confirmed that selective photoexcitation at the near‐IR absorption bands of PBI.− is unable to produce PBI2−. [ABSTRACT FROM AUTHOR]

Singlet exciton fission (SF) is a spin-allowed process whereby two triplet excitons are created from one singlet exciton. This phenomenon can offset UV photon energy losses and enhance the overall efficiency in photovoltaic devices. For this purpose, it requires photostable commercially available SF materials. Excited state dynamics in pure perylene film, ease of commercial production, is studied by time-resolved fluorescence and femtosecond transient absorption techniques under different photoexcitation energies. In film, polycrystalline regions contain perylene in H-type aggregate form. SF takes place from higher excited states of these aggregates in ultrafast time scale < 30 fs, reaching a triplet formation quantum yield of 108%. Moreover, at λex = 450 nm singlet fission was detected as a result of two-quantum absorption. Other competing relaxation channels are excimer (1 ps) and dimer radical cation formation (< 30 fs). Excimer radiatively relaxes within 19 ns and radical cation recombines in 3.2 ns. Besides, exciton self-trapping by crystal lattice distortions occurs within hundreds of picosecond. Our results highlight potential of simple-fabricated perylene films with similar properties as high-cost single crystal in SF based photovoltaic applications. [ABSTRACT FROM AUTHOR]

Quasi-two-dimensional (2D) layered perovskites have lately attracted extensive research interest due to their superior environmental stability to their three-dimensional (3D) counterparts. While solar cells based on 2D layered perovskites have achieved remarkably high efficiencies over 18%, their intrinsic carrier dynamics after photoexcitation remain obscure due to the presence of bound excitons and the coexistence of multiple layered perovskite phases. Discrepancies were found in the interpretations of the reported ultrafast processes with hundreds of femto- or pico-second time scales in previous literature via transient absorption measurements. Here, carrier dynamics in (PEA)2(MA)2Pb3I10 (PEA = C6H5(CH2)2NH3; MA = CH3NH3) 2D layered perovskite was systematically studied through a combination of transient absorption and time-resolved photoluminescence measurements with femtosecond time resolution. The ultrafast sub-ps (100–400 fs) carrier dynamics is unambiguously demonstrated due to hot carrier relaxation. Carrier transport occurs from small n layered perovskites to their surrounding 3D like (large n) perovskite via energy transfer on the time scale of 2–300 ps. These results provide valuable information on the intrinsic carrier dynamics in 2D layered perovskites which would be helpful in further improving the device performance. [ABSTRACT FROM AUTHOR]

The two-photon-induced singlet fission was observed in rubrene single crystal and studied by use of femtosecond pump-probe spectroscopy. The location of two-photon excited states was obtained from the nondegenerate two-photon absorption (TPA) spectrum. Time evolution of the two-photon-induced transient absorption spectra reveals the direct singlet fission from the two-photon excited states. The TPA absorption coefficient of rubrene single crystal is 52 cm/GW at 740 nm, as obtained from Z-scan measurements. Quantum chemical calculations based on time-dependent density functional theory support our experimental data. [ABSTRACT FROM AUTHOR]

In solar thermal utilization, effective photo capture and photothermal conversion are crucial. Improving the nonradiative transition rate of photoexcited electrons is important to enhance the photothermal conversion efficiency and develop efficient solar thermal utilization. Herein, we designed a new kind of light absorption‐enhanced and efficient photothermal conversion material, namely, Fe2O3‐functionalized reduced graphene oxide (Fe2O3–RGO). On the basis of the selective absorption (350–560 nm) of Fe2O3 and the synergistic effect of RGO on the quenching energy transfer of the excited state of Fe2O3 and ultrafast nonradiative thermal decay of RGO, the optical absorption capacity, and photothermal conversion efficiency of the composites were effectively improved. Fe2O3–RGO can be successfully applied to photothermal conversion phase change materials and seawater solar desalination, showing excellent photothermal conversion ability and application prospect. [ABSTRACT FROM AUTHOR]

Fundamental photocatalytic limitations of solar CO2 reduction remain due to low efficiency, serious charge recombination, and short lifetime of catalysts. Herein, two‐dimensional graphitic carbon nitride nanosheets with nitrogen vacancies (g‐C3Nx) located at both three‐coordinate N atoms and uncondensed terminal NHx species were prepared by one‐step tartaric acid‐assistant thermal polymerization of dicyandiamide. Transient absorption spectra revealed that the defects in g‐C3N4 act as trapped states of charges to result in prolonged lifetimes of photoexcited charge carriers. Time‐resolved photoluminescence spectroscopy revealed that the faster decay of charges is due to the decreased interlayer stacking distance in g‐C3Nx in favor of hopping transition and mobility of charge carriers to the surface of the material. Owing to the synergic virtues of strong visible‐light absorption, large surface area, and efficient charge separation, the g‐C3Nx nanosheets with negligible loss after 15 h of photocatalysis exhibited a CO evolution rate of 56.9 μmol g−1 h−1 under visible‐light irradiation, which is roughly eight times higher than that of pristine g‐C3N4. This work presents the role of defects in modulating light absorption and charge separation, which opens an avenue to robust solar‐energy conversion performance. Between the sheets: Graphitic carbon nitride (g‐C3N4) nanosheets rich in nitrogen defects were obtained by one‐step tartaric acid‐assisted thermal polymerization of dicyandiamide. The defects in g‐C3N4 act as charge trapped states to prolong the lifetime of charge carriers, and the decreased interlayer distance caused by nitrogen vacancies favors of charge transport and mobility to the surface (see figure). Deficient g‐C3N4 exhibits the enhanced photocatalytic activity in CO2 reduction. [ABSTRACT FROM AUTHOR]

Electron transfer in solid is crucial to switchable magnetic, electrical, optical and mechanical properties. However, it is a formidable challenge to control electron-transfer behaviors via manipulation of crystalline phases, especially through dynamic crystalline transformation. Herein, three crystalline phases of an {Fe2Co2} compound were obtained via enhancement of intermolecular π···π interactions inducing successive single-crystal-to-single-crystal transformations, from solvated 1·2CH3OH·4H2O, to desolvated 1 and its polymorph 1a accompanying electron transfer. 1·2CH3OH·4H2O showed thermally induced reversible intermetallic electron transfer in mother liquor. No electron-transfer behavior was observed in 1. 1a showed reversible intermetallic electron transfer upon thermal treatment or alternative irradiation with 808- and 532-nm lasers at cryogenic temperatures. The electron-transfer behaviors significantly change the magnetic and optical properties, providing a strategy to realize different electron-transfer behaviors and switchable functions via π···π interactions manipulated dynamic crystalline transformation. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs), ZIF-67, with different morphologies were synthesized via a solvent-induced method at room temperature. The photocatalytic performances towards the reduction of CO2 were evaluated by using ZIF-67 materials as cocatalysts cooperating with a ruthenium-based complex as the photosensitizer. It has been demonstrated that the two-dimensional ZIF-67 with a leaf-like morphology exhibited the best photocatalytic activity and stability due to the highest CO2 adsorption capability and efficient electron transfer from the excited [Ru(bpy)3]2+ to ZIF-67. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) perovskites with the general formula APbX4 are attracting increasing interest as solution processable, white-light emissive materials. Recent studies have shown that their broadband emission is related to the formation of intra-gap colour centres. Here, we provide an in-depth description of the charge localization sites underlying the generation of such radiative centres and their corresponding decay dynamics, highlighting the formation of small polarons trapped within their lattice distortion field. Using a combination of spectroscopic techniques and first-principles calculations to study the white-light emitting 2D perovskites (EDBE)PbCl4 and (EDBE)PbBr4, we infer the formation of Pb23+, Pb3+, and X2− (where X = Cl or Br) species confined within the inorganic perovskite framework. Due to strong Coulombic interactions, these species retain their original excitonic character and form self-trapped polaron–excitons acting as radiative colour centres. These findings are expected to be relevant for a broad class of white-light emitting perovskites with large polaron relaxation energy. [ABSTRACT FROM AUTHOR]

Multiple exciton generation (MEG) is a promising process to improve the power conversion efficiency of solar cells. PbSe quantum dots (QDs) have shown reasonably high MEG quantum yield (QY), although the photon energy threshold for this process is still under debate. One of the reasons for this inconsistency is the complicated competition of MEG and hot exciton cooling, especially at higher excited states. Here, we investigate MEG QY and the origin of the photon energy threshold for MEG in PbSe QDs of three different sizes by studying the transient absorption (TA) spectra, both at the band gap (near infrared, NIR) and far from the band gap energy (visible range). The comparison of visible TA spectra and dynamics for different pump wavelengths, below, around and above the MEG threshold, provides evidence of the role of the Σ transition in slowing down the exciton cooling process that can help MEG to take over the phonon relaxation process. The universality of this behavior is confirmed by studying QDs of three different sizes. Moreover, our results suggest that MEG QY can be determined by pump–probe experiments probed above the band gap. [ABSTRACT FROM AUTHOR]

A novel hole-transporting molecule (F101) based on a furan core has been synthesized by means of a short, high-yielding route. When used as the hole-transporting material (HTM) in mesoporous methylammonium lead halide perovskite solar cells (PSCs) it produced better device performance than the current state-of-the-art HTM 2,2′,7,7′-tetrakis-( N, N-di- p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). The F101-HTM-based device exhibited both slightly higher Jsc (19.63 vs. 18.41 mA cm−2) and Voc (1.1 vs. 1.05 V) resulting in a marginally higher power conversion efficiency (PCE) (13.1 vs. 13 %). The steady-state and time-resolved photoluminescence show that F101 has significant charge extraction ability. The simple molecular structure, short synthesis route with high yield and better performance in devices makes F101 an excellent candidate for replacing the expensive spiro-OMeTAD as HTM in PSCs. [ABSTRACT FROM AUTHOR]

Transient absorption properties of aqueous graphene oxide (GO) have been studied by use of femtosecond pump–probe spectroscopy. Excited state absorption and photobleaching are observed in the wide spectral range. The observed fast three lifetime components are attributed to the absorption of upper excited states and localized states, which is confirmed by both laser induced absorption and transmission kinetics. The longest time component is assigned to the lowest excited state of GO, which mainly originates from the sp2 domains. With the increase of the excitation power, two-quantum absorption occurs, which results in an additional rise-time component of the observed transients. [ABSTRACT FROM AUTHOR]

Few-layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low-wavenumber Raman peak of ~120 cm−1 and a phonon-induced oscillation in the kinetic curve of electron-phonon relaxation process have been observed, respectively. The Raman peak is assigned to the low-wavenumber out-of-plane optical mode in the few-layer graphene. The phonon band shows an asymmetric shape, a consequence of so-called Breit-Wigner-Fano resonance, resulting from the coupling between the low-wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low-wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The transient differential transmission (ΔT/T) spectra of graphene were obtained from 380 (3.3 eV) to 670 nm (1.9 eV). The intraband carrier equilibration by carrier-carrier scattering occurred within 60 fs. The subsequent carrier relaxation process was governed by carrier-optical phonon scattering and had linear dependence on the probe photon energy (Epr); lifetimes ranged from 180 to 90 fs for Epr from 2.1 to 2.8 eV. Negative ΔT/T signals in kinetic curves were discussed and assigned to thermal diffusion and shrinkage of band separation caused by lattice heating. [ABSTRACT FROM AUTHOR]

Herein, we successfully applied a facile in-situ solid-state synthesis of conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) as a HTM, directly on top of the perovskite layer, in conventional mesoscopic perovskite solar cells (PSCs) (n-i-p structure). The fabrication of the PEDOT film only involved a very simple in-situ solid-state polymerisation step from a monomer 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) made from a commercially available and cheap starting material. The ultraviolet photoelectron spectroscopy (UPS) demonstrated that the as-prepared PEDOT film possesses the highest occupied molecular orbital (HOMO) energy level of −5.5 eV, which facilitates an effective hole extraction from the perovskite absorber as confirmed by the photoluminescence measurements. Optimised PSC devices employing this polymeric HTM in combination with a low-cost vacuum-free carbon cathode (replacing the gold), show an excellent power conversion efficiency (PCE) of 17.0% measured at 100 mW cm−2 illumination (AM 1.5G), with an open-circuit voltage (Voc) of 1.05 V, a short-circuit current density (Jsc) of 23.5 mA/cm2 and a fill factor (FF) of 0.69, respectively. The present finding highlights the potential application of PEDOT made from solid-state polymerisation as a HTM for cost-effective and highly efficient PSCs. [ABSTRACT FROM AUTHOR]